An active matrix type liquid crystal display device, placing TFT on a display cell substrate in a matrix form, is a device which makes possible mass storage display of high quality. The device has been intensively applied to televisions and the like.
A conventional process for manufacturing TFT array substrates suitable for liquid crystal televisions will be illustrated based on FIGS. 14(A) and (B). FIG. 14(A) shows a plane view of one picture element of the TFT array manufactured by a conventional process. FIG. 14(B) shows an X--X' sectional view of the picture element. A metal layer such as Al and the like is formed on a transparent insulation substrate 70 and patterned by photo-etching to form a gate electrode bar 71. A gate insulation layer 72 made from an oxide film or a nitride film and a semiconductor layer 73 made from Si, CdS and the like are successively laminated and the semiconductor layer 73 is etched. On the semiconductor 73, a transparent electrode layer is laminated and etched to form a source electrode bar 76 and a drain electrode or display electrode 77.
As described above, the conventional process requires at least two mask alignments, because it employs at least three for etching. This complicates the manufacturing process of the TFT array substrate and causes high cost and low yield.